The Role of Achiral Pyrazolidinone Templates in Enantioselective Diels-Alder Reactions: Scope, Limitations, and Conformational Insights
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- 作者：Mukund P. Sibi ; Levi M. Stanley ; Xiaoping Nie ; Lakshmanan Venkatraman ; Mei Liu ; Craig P. Jasperse
- 刊名：Journal of the American Chemical Society
- 出版年：2007
- 出版时间：January 17, 2007
- 年：2007
- 卷：129
- 期：2
- 页码：395 - 405
- 全文大小：275K
- 年卷期：v.129,no.2(January 17, 2007)
- ISSN：1520-5126
文摘
We have evaluated the role of achiral pyrazolidinone templates in conjunction with chiral Lewisacids in room temperature, enantioselective Diels-Alder cycloadditions. The role of the fluxional N(1)substituent was examined, with the bulky 1-naphthylmethyl group providing enantioselectivities up to 99%ee, while templates with smaller fluxional groups gave lower selectivities. High selectivities were alsoobserved in reactions of 7d with chiral Lewis acids derived from relatively small chiral ligands, suggestingthe pyrazolidinone templates are capable of relaying stereochemical information from the ligand to thereaction center. Lewis acids capable of adapting square planar geometries, such as Cu(OTf)2, Cu(ClO4)2,and Pd(ClO4)2, were found to be particularly effective at providing high selectivities. Additionally, substitutionat the C-5 position of the pyrazolidinone templates has been shown to be critical for optimal selectivity.Reactions of the optimal pyrazolidinone appended with a number of common dienophiles and various dienesdemonstrate the utility of this achiral template. Furthermore, catalytic loadings could be lowered to 2.5 mol% with essentially no loss in selectivity. 
- interactions were evaluated as a means to explain the unusuallyhigh selectivity observed at room temperature. Finally, non-C2-symmetric ligands were employed as a testto determine if chiral relay was operative.
- interactions were evaluated as a means to explain the unusuallyhigh selectivity observed at room temperature. Finally, non-C2-symmetric ligands were employed as a testto determine if chiral relay was operative.